Potential of Dairy Coproducts in Wood Modification: Dimensional Stability and Mechanical Properties

Chemical modification of wood using citric acid and polyols is a promising biosourced route to improve dimensional stability. However, conventional polyols can be costly and derived from food-grade resources. Dairy coproducts such as whey and whey ultrafiltration permeate represent abundant, low-value side streams rich in carbohydrates and nitrogenous compounds that could act as sustainable polyol substitutes. Whey ultrafiltration permeate was already investigated for wood modification. However, sweet whey has not been investigated to date. In addition, no direct comparison with wood modified with citric acid has been reported. Such a comparison would help to better understand the mode of action of these dairy coproducts and what their advantages would be in comparison to citric acid, which is already well‑known in wood modification. Therefore, this study aimed to evaluate the potential of these coproducts as a source of polyol, alone or combined with citric acid, to improve the physical and mechanical performances of Scots pine (Pinus sylvestris L.) sapwood. Full‑cell impregnation followed by curing at 160 °C was used to promote in situ polycondensation reaction between citric acid and components of dairy coproducts. Dairy coproduct-based formulations containing citric acid achieved high fixation after water leaching, positive cell wall bulking, and stable anti-swelling efficiency, confirming the formation of hydrophobic ester‑rich polymer within the wood matrix. Moisture content at 95% RH decreased by ~ 32% relative to untreated wood. Surface hardness increased by 30% while bending strength was reduced less compared with citric acid alone. Scanning electron microscopy and Fourier transform infrared spectroscopy confirmed extensive polymer deposition and ester formation within the wood matrix. These results demonstrate the promising potential of dairy coproducts as circular, low-cost reactants for biosourced wood modification.